A student places a piece of potato tissue into a solution that has a lower water potential than the cytoplasm of the potato cells. Explain what happens to the potato cells over time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Potato cells contain a large central vacuole filled with cell sap. When potato tissue is placed in solutions of different water potentials, the cells can gain or lose water.
Model answer (5 marks)
Water moves out of the potato cells by osmosis. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Because the external solution has a lower water potential than the cytoplasm/vacuole, water leaves the cells. The vacuole and cytoplasm shrink, the cell membrane pulls away from the cell wall (plasmolysis). The potato tissue becomes flaccid and loses its firmness as the cells are no longer turgid.
Examiner tips
- Define osmosis and relate it to water potential differences
- Show the direction of water flow (outside to inside)
- Explain the physical change in the cell (plasmolysis)
- Link the loss of turgor to the tissue becoming soft
Common mistakes
- Confusing water potential with solute concentration
- Saying water moves into the cells instead of out
- Failing to mention plasmolysis or loss of turgor
Mark scheme (5 marks)
- Water moves out of the potato cells (by osmosis)
- Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential (through a partially permeable membrane)
- The water potential of the external solution is lower than that of the cell cytoplasm / vacuole, so there is a net movement of water out of the cells
- The vacuole and cytoplasm shrink / the cell becomes plasmolysed OR the cell membrane pulls away from the cell wall
- The potato tissue becomes flaccid / soft / loses its firmness (because cells are no longer turgid)
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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